Turbofan engines are typically associated with running power plants or powering airplanes. With respect to airplanes, aeromechanical instabilities such as flutter may catastrophically lead to blade failure. Flutter is characterized by a resonance or elastic deformation of the turbofan blades generated by the coupling of the aerodynamics and the structural dynamics of the blades. The blades have natural and associated harmonic frequencies of resonance which are based on the blade structure or configuration. An axial turbomachinery blade is associated with structural mode shapes which are the natural patterns and frequencies in which the blade deflects and resonates when excited. A blade has more than one mode shape and each mode shape resonates at a particular frequency. When an instability such as flutter occurs, it is usually associated with one particular structural mode excited by the coupling with the unsteady aerodynamics. It is therefore vitally important to detect these instabilities in aeropropulsion compression systems and to dampen the instability dynamics to prevent such imminent blade failure.
Such aeromechanical instabilities impose significant constraints on the design and development of modern aero-engines. As shown in FIG. 8, of particular concern is flutter which has many occurances on a fan's pressure ratio (ordinate) versus mass flow (abscissa) operating map 400. As shown in FIG. 8, curves 402 and 404 respectively correspond to the operating line and flutter lines of the operating map. The constraint on blade design is to keep the flutter boundaries outside of the operating envelope of the engine.
FIGS. 1a and 1b illustrate (in exaggerated form) blade resonance or energy waves generated in a turbofan 200 having eight blades 202, 204, 206, 208, 210, 212, 214 and 216. The blades 200-216 are shown in solid form corresponding to a non-deflected state, and the blades 204-208 and 212-216 are also shown in phantom form corresponding to a deflected state during a resonance or elastic deformation of the blades which may arise due to flutter during blade rotation. FIG. 1b maps the degree of deformation of each blade during an instant of time where the amount of blade deformation in the direction of blade rotation is a positive value and the amount of blade deformation in the direction opposite to blade rotation is a negative value.
At an instant of time during rotation of the turbofan 200 in the clockwise direction, the blade 202 is shown in FIG. 1a to have no deformation which corresponds to a deformation value of zero units for the blade 202 as mapped in FIG. 1b. The blade 204 is shown in FIG. 1a to have a slight deformation in the direction of rotation which corresponds to a positive deformation of one unit for the blade 204 as mapped in FIG. 1b. The blade 206 is shown in FIG. 1a to have an even greater deformation relative to the blade 204 in the direction of rotation which corresponds to a positive deformation of two units for the blade 206 as mapped in FIG. 1b. The blade 208 is shown in FIG. 1a to have the same deformation as the blade 204 which corresponds to a positive deformation of one unit for the blade 208 as mapped in FIG. 1b.
The blade 210 is shown in FIG. 1a to have no deformation which corresponds to a deformation value of zero units for the blade 210 as mapped in FIG. 1b. The blade 212 is shown in FIG. 1a to have a slight deformation in a direction opposite to blade rotation which corresponds to a negative deformation of one unit for the blade 212 as mapped in FIG. 1b. The blade 214 is shown in FIG. 1a to have an even greater deformation relative to the blade 212 in the direction opposite to blade rotation which corresponds to a negative deformation of two units for the blade 214 as mapped in FIG. 1b. The blade 216 is shown in FIG. 1a to have the same deformation as the blade 212 which corresponds to a negative deformation of one unit for the blade 216 as mapped in FIG. 1b. The resonance pattern shown in FIGS. 1a and 1b correspond at an instant of time to one sinusoidally shaped cycle of deformation of the blades as seen along a 360.degree. path circumaxially about the turbofan 200. However, other excitation patterns characterized by zero or multiple cycles contribute to flutter in aerocompression systems.
Flutter in axial turbomachinery typically occurs in specific nodal diameters dependent on the particular geometry of the turbomachinery. A nodal diameter is the wave number of the sinusoid that the blade deflection pattern represents. FIGS. 2a-2c illustrate various nodal diameters of the blade deflection pattern of turbofan blades. The length and direction of the arrows in each figure define respectively the degree and direction (positive or negative direction) of the turbofan blades as viewed at an instant of time about the rotational axis of the turbofan from a start point (0.degree.) to the end point (360.degree.). As is evident, the start and end points are the same physical position. FIG. 2a illustrates a 0th nodal diameter pattern 227 of arrows 229 diagrammatically representing the direction and degree of blade deflection in which each turbofan blade exhibits no blade deflection or the same amount of blade deflection with respect to one another when viewed at an instant of time at any point around the axis of rotation of the turbofan. FIG. 2b shows a 1st nodal diameter deflection pattern 231 of arrows 233 representing blade deflection at an instant of time in which the turbofan blades as viewed circumaxially about the turbofan exhibit a single cycle generally sinusoidal wave pattern. Such nodal diameter deflection patterns illustrate the general resonance deflection pattern of a turbofan in a manner which is independent of the actual number of blades comprising the turbofan. As can be seen, the 1st nodal diameter deflection pattern of FIG. 2b corresponds to the deflection pattern embodied by the eight turbofan blades in FIG. 1a. FIG. 2c illustrates a 3rd nodal diameter deflection pattern 235 of arrows 237 representing blade deflection at an instant of time in which the turbofan blades as viewed circumaxially about the turbofan axis of rotation exhibit a three cycle generally sinusoidal wave pattern. The structural mode shape is the natural pattern in which an axial turbomachinery blade deflects and resonates when excited. A blade has more than one mode shape and each mode shape resonates at a particular frequency. When flutter occurs, it usually is associated with one particular structural mode. Flutter is difficult to predict analytically and expensive to investigate experimentally. Consequently, flutter is often encountered only in the final phases of engine development leading to expensive delays and often forcing a degradation in overall system performance.
In response to the foregoing, it is an object of the present invention to overcome the drawbacks and disadvantages of prior art apparatus and methods for preventing aeromechanical instabilities in aero-engines.